Hacking Honda Odyssey 2007 RES DVD-player

Some schematics and connector information

http://dccharacter.blogspot.com/2016/03/hacking-honda-odyssey-2007-res-dvd_27.html

My weekend project was to actually build a board (with good connectors, which is much safer than having an unisolated wires directly from the car battery hanging right above the ground - which is pretty much everywhere in a car). SSSO I did build an ugly board, and it didn't work initially, but took very little troubleshooting to start. Than interesting things started to happen. The board worked in the car. Until it wouldn't. It took about 3-4 minutes for it to stop working. As the only display I had was a bi-color LED, I only knew that it receives a request from the head, but thinks it's not a properly formatted frame. What the hell? Can the head figure out I'm messing with it? Are there any requests that it sends that I miss and do not respond to so it can tell there's something instead of DVD? Why would anyone took such a precaution? I mean it's just a car and just a DVD player...

Spent most of the day to figure that out. Tried many things and was almost ready to switch on my soldering iron again... But. But. I luckily decided to exhaust my firmware possibilities first. And what I figured out, if you don't check for the correctness of the frame, but just respond to it in a way as if it would be a normal frame, it works. Interesting! I started digging into my code and figured I made a mistake in one of the places where check sum was calculated. IT ONLY USES 7 LSBs of the sum! Fixed that and it worked as a charm!